Flight of a Cytidine Deaminase Complex with an Imperfect Transition State Analogue Inhibitor: Mass Spectrometric Evidence for the Presence of a Trapped Water Molecule

Cytidine deaminase (CDA) binds the inhibitor zebularine as its 3,4-hydrate (Kd ∼ 10–12 M), capturing all but ∼5.6 kcal/mol of the free energy of binding expected of an ideal transition state analogue (Ktx ∼ 10–16 M). On the basis of its entropic origin, that shortfall was tentatively ascribed to the trapping of a water molecule in the enzyme–inhibitor complex, as had been observed earlier for product uridine [Snider, M. J., and Wolfenden, R. (2001) Biochemistry 40, 11364–11371]. Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) of CDA nebularized in the presence of saturating 5-fluorozebularine reveals peaks corresponding to the masses of E2Zn2W2 (dimeric Zn-CDA with two water molecules), E2Zn2W2Fz, and E2Zn2W2Fz2, where Fz represents the 3,4-hydrate of 5-fluorozebularine. In the absence of an inhibitor, E2Zn2 is the only dimeric species detected, with no additional water molecules. Experiments conducted in H218O indicate that the added mass W represents a trapped water molecule rather than an isobaric ammonium ion. This appears to represent the first identification of an enzyme-bound water molecule at a subunit interface (active site) using FTICR-MS. The presence of a 5-fluoro group appears to retard the decomposition of the inhibitory complex kinetically in the vapor phase, as no additional dimeric complexes (other than E2Zn2) are observed when zebularine is used in place of 5-fluorozebularine. Substrate competition assays show that in solution zebularine is released from CDA (koff > 0.14 s–1) much more rapidly than is 5-fluorozebularine (koff = 0.014 s–1), despite the greater thermodynamic stability of the zebularine complex.